Monin–Obukhov length as a cornerstone of the SEBAL calculations of evapotranspiration

Abstract

Summary The paper looks at the pivotal role that the Monin–Obukhov length (L) can play within the sensible heat balance method for calculating evapotranspiration from remotely sensed multispectral data. It focuses on the possibility of using L as a cornerstone parameter for calculations of sensible heat flux from the pixel data. The paper shows that L can be separated from the other unknowns and can be found either by means of any general one-dimensional root finding method, or iteratively, starting with L ≃ −∞. A simple algorithm is suggested to reduce the number of iterations. The paper goes on to show that all other output parameters can be calculated directly from the value of L that is established, and an analytical expression for the evaporative fraction found. A novel approach is proposed that allows a close approximation of the evaporative fraction to be derived directly without the need for time-consuming iterations within the main calculation step. Estimates of evapotranspiration from cotton in Southern Kazakhstan showed that the approximation performed well when compared with estimates of ETcrop made using an iterative approach. An experimental approach for calculating E24 is described for use in areas with minimal cloud cover. Comparison of SEBAL estimated Etc with calculated Et0 for the short season cotton crop in Southern Kazakhstan indicated that for the actively transpiring cotton crop the crop factors were the same as those determined experimentally for low short season cotton in Kansas by Hunsaker [Hunsaker, D.J., 1999. Basal crop coefficients and water use for early maturity cotton. Trans. ASAE 42(4), 927–936].